School Flower Garden Yearly Calendar provides advice on when to sow, when to grow and when to harvest. If you ever wanted to grow annuals and perennials organically with your students but just wasn’t sure how to proceed, then look no further.As large and diverse as North America is, it’s impossible to work out a schedule that is accurate in every single area. So, to be diplomatic, School Flower Garden Yearly Calendar has been timed around Zone 4, with a last frost date roughly around May 20th.